1. Field of the Invention
The present invention relates to a B-ISDN (Broadband ISDN) transmission device.
2. Description of the Related Art
In a public transmission network, telephony (speech) signals or various data information in an STM (Synchronous Transfer Mode) format are transmitted and handled within the infrastructure of an SDH (Synchronous Digital Hierarchy) or a SONET (Synchronous Optical NETwork). Service signals in the STM format are accessed by each transmission device at a level of an STS (Synchronous Transport Signal: 51.84 MHz) or of a VT (Virtual Tributary: 1.728 MHz), which are the frame formats of SONET. Additionally, an external interface uses a variety of signal formats such as OC-48 (optical signal: 2.4 GHz), OC-12 (optical signal: 600 MHz), OC-3 (optical signal: 150 MHz), DS3 (electric signal: 44.736 MHz), DS1 (electric signal: 1.544 MHz), etc.
Furthermore, data information services in an ATM (Asynchronous Transfer Mode) signal format have been made practical in recent years so as to effectively use the bandwidth of a transmission device or a transmission line. From the viewpoint of future network operations, it is demanded that terminal repeater equipment or an exchange not only carries ATM signals but also can access ATM signals as well as STM signals. That is, an ATM switching capability and diversified data interface capabilities such as 10Base-T, 100Base-T, FrameRelay, DS1-UNI, DS3-UNI, OC3UNI, etc. are required.
For an implementation, new services such as an ATM service, etc. are introduced in stages while using existing services, in most cases. Especially, in a data-service-related industry, devices are used in a variety of ways due to the entries of various service providers. Accordingly, the upgradability and the flexibility of the devices are demanded.
A conventional switching transmission device which can accommodate both the ATM and the STM is disclosed, for example, by the Japanese Patent Laid-Open Publication No. 1-148000. The STM/ATM hybrid switching transmission device disclosed by this publication will be briefly explained below.
FIG. 1 exemplifies the conventional network where the STM and the ATM are mixed.
In this figure, STM and ATM signals can be carried in STS-1xc3x97N units over a single optical fiber by an ADM (Add Drop Multiplexer) or by a SONET MUX (SONET Multiplexer), as long as the signals use the SONET frame format as a physical transmission medium. However, an ATM access capability is not included in the conventional network. When a communication between terminals using the ATM is attempted to be made, an ATM exchange must be passed through or an ATM MUX (ATM Multiplexer) must include an ATM switching capability. In this case, however, the bandwidth of a transmission line such as an optical fiber, etc. is wasted, and many terminals and multiplexing devices are required.
That is, when terminals 201 and 202 of FIG. 1, which are ATM terminals, attempt to make a communication, they can make a communication on a path from the terminal 201xe2x80x94an ATM MUX 205xe2x80x94the terminal 202, if the ATM MUX 205 has a switching capability. However, if the ATM MUX 205 does not have a switching capability, the ATM cell transmitted from the terminal 201 must reach a BB exchange (BroadBand exchange) 203 which is an ATM exchange via the ATM MUX 205 and SONET ADM/MUXes 204 and 206 as indicated by a path (1) shown in FIG. 1. The BB exchange switches the ATM cell, which is made to reach the terminal 202 again via the SONET ADM/MUXes 206 and 204 and the ATM MUX 205.
As described above, in the conventional network where the ATM and the STM are mixed, a trunk transmission line must be used for the communication between the terminals 201 and 202, which are connected to the single ATM MUX 205, if the ATM MUX 205 does not have a switching capability. Therefore, the transmission bandwidth of the optical fiber of the trunk line is wasted. Additionally, if the ATM MUX 205 is equipped with the switching capability, its hardware configuration becomes complicated and the cost increases. Furthermore, since all ATM MUXes which accommodate ATM terminals must be equipped with this capability in a similar manner, the cost of the entire network rises.
Explained below is the outline of the conventional STM/ATM hybrid switching transmission device disclosed by the above described publication. For more details, please refer to this publication.
FIG. 2 is a block diagram exemplifying the configuration of the conventional STM/ATM hybrid switching transmission device.
In this figure, 191 indicates a highway and a plurality of highways 191 are arranged. In each of the plurality of highways 191, STM and ATM information are multiplexed. 192 and 193 indicate time slot phase switching units. 195 indicates a time division slot switching device. 197 indicates a time-division switching device. The time-division switching device 197 includes the time slot phase switching units 192 and 193, and the time-division slot switching device 195.
Each of the time slot phase switching units 192 is arranged for each incoming highway 191, while each of the time slot phase switching units 193 is arranged for each outgoing highway 191. These time slot phase switching units 192 and 193 are intended to switch between the phases of time slots of each of the highways 191.
The time-division slot switching device 195 is intended to switch between the time slots of the highways 191.
This time-division slot switching device 195 separates the STM and ATM information, and switches between the time slots of the highways 191 for the STM information. Furthermore, the time-division slot switching device 195 has a capability for multiplexing and switching the STM and ATM information upon receipt of the output from an asynchronous transfer mode information switching device (ATM switching device).
196 indicates an ATM switching device which is intended to switch the ATM information separated by the time-division slot switching device 195 included in the time-division switching device 197.
Since the capacity of the STM switch unit of each time slot phase switching unit 192 and the time-division slot switching device 195, and the capacity of the ATM switch unit of the ATM switching device 196 are fixed in this configuration, the hardware size becomes larger as the respective capacities of the STM and the ATM switch units increase. At the same time, the efficiency decreases. If this configuration is used as an STM or an ATM dedicated machine, it is not cost-effective due to the inclusion of unnecessary portions. Additionally, because the minimum unit of switching is fixed in the STM switch unit, the hardware configuration becomes complicated if switching is attempted to be made also on a low level such as DS1 (1.544 MHz). Furthermore, if a redundant configuration is adopted, the entire hardware must be duplexed. As a result, the reliability degrades because the entire configuration is switched even at the time of the occurrence of a partial problem.
Conventionally, a terminal and an exchange which handle STM and ATM signals are respectively implemented as different devices. There are several methods for handling STM and ATM signal services by combining these devices, and for accessing/switching an STM signal by forcibly putting the STM signal into ATM cells, in the hardware which also accesses also ATM signals.
With the conventional methods, however, a node (device) within a network becomes redundant in order to realize both the coexistence with the network composed of an existing SONET ADM (Add-Drop Multiplexer) for an STM or of an ATM access node/exchange, and the support of various service interfaces of respective devices. As a result, an inefficient network is configured or a device configuration becomes complicated in order to convert an STM signal into ATM cells, which are signals of a different type. Therefore, it is impossible to reduce the size and cost of the device.
Additionally, with the configuration of the STM/ATM hybrid switching transmission device which can switch both STM and ATM signals, such as disclosed by the Japanese Patent Laid-Open Publication No. 1-148000, conventional STM services such as SONET, DS1, DS3, etc., and data services such as 10Base-T, 100Base-T, Frame Relay, etc. are difficult to flexibly support various interfaces of ATM services such as OC3-UNI, DS3-UNI, etc. without leaving a hardware resource unused. Similarly, the balance between the capacities of STM and ATM switches varies depending on a service provider, and changes in large increments. Therefore, the hardware size which satisfies all of the above described requirements becomes larger, so that higher costs and a bigger space for installation are required. Especially, the cost of an ATM switch varies depending on its capacity. Additionally, it is difficult to maintain the compatibility between the STM and the ATM switches also from the viewpoint of maintenance for making STM and ATM accesses.
An object of the present invention is to provide a compact and low-cost device which accommodates an ATM signal access in an STM accessing device arranged in a conventional STM network while keeping the coexistence with the STM accessing device, efficiently performs STM and ATM signal processes, and can flexibly cope with a service type or a capacity change.
A transmission device according to the present invention includes an interface unit for supporting a synchronous communications network, an asynchronous communications network, and a data service, etc.; a switching unit for switching all or any of the frame level of a synchronous communications network, the frame level of a low layer of the synchronous communications network, and the frame level of an asynchronous communications network; and a centralized control unit for performing various controls including the switching control in the case where the interface and switching units are redundantly configured. Additionally, this transmission device serves as a transmission device dedicated to a synchronous communications network, a transmission device dedicated to an asynchronous communications network, or a synchronous/asynchronous hybrid switching transmission device by combining the interface unit, the switching unit, and the centralized control unit.
According to the present invention, the switching unit switches all or any of the frame level of a synchronous communications network, the frame level of a low layer of the synchronous communications network, and the frame level of an asynchronous communications network. Accordingly, the capability for switching at a frame level of the synchronous communication network is implemented, and at the same time, it becomes possible to make switching in frame units other than the frame level of the synchronous communications network. Here, the frame level of the synchronous communications network is, for example, a SONET STS frame. The frame level of the low layer of the synchronous communications network is, for example, a VT level. The frame level of the asynchronous communications network is an ATM cell level.
Furthermore, there is no need to newly arrange a special interface by using the frame level of a synchronous communications network as a shared interface between the units for switching respective frame levels of the synchronous communications network. Therefore, signal processing can be performed with simple circuitry based on the ITU-T recommendations.